1. Field of the Invention
The present invention relates to an image forming apparatus in which a toner image on an image carrier such as a photoconductor is transferred onto a front surface of an endless belt member or onto a recording material held on the belt, at a transfer nip formed where the image carrier abuts the belt.
2. Description of the Related Art
An image forming apparatus having the above-described configuration may employ an intermediate transfer method, for example. In such an image forming apparatus, a toner image corresponding to image information is formed on a photoconductor, and the toner image is transferred onto an intermediate transfer belt (endless belt member) by a primary transfer process. Then, the toner image on the intermediate transfer belt is transferred onto a sheet which is a recording material by a secondary transfer process, thereby forming an image on the sheet. In such an image forming apparatus, in the entire region on the back surface (the inner side of the loop) of the intermediate transfer belt which is a belt member, a primary transfer roller is caused to abut a region behind a primary transfer nip to apply a primary transfer bias, thus forming a primary transfer electric field between the photoconductor and the intermediate transfer belt. The shaft of the primary transfer roller is supported by bearings made of resin etc., and the primary transfer roller is caused to rotate by the intermediate transfer belt.
A conductive foam rubber roller, etc., is generally used as the primary transfer roller. The conductive foam rubber roller has a foamable conductive rubber layer provided around a cored bar, as described in patent document 1, for example.
The conductive foam rubber roller is expensive, and thus increases the cost of the apparatus. In particular, a tandem-type full-color image forming apparatus requires four primary transfer rollers, which significantly increases the cost of the apparatus.
Patent document 2 describes an image forming apparatus which includes a metal roller used as the primary transfer roller. Metal rollers cost less than conductive foam rubber rollers, and therefore the apparatus costs less than that with a conductive foam rubber roller.
Patent Document 1: International Application Publication No. WO02/056119
Patent Document 2: Japanese Laid-Open Patent Application No. 2006-072247
However, when a metal roller is used as the primary transfer roller, the following problem arises. That is, the surface of a metal roller has a low friction coefficient, and therefore tends to slip on the intermediate transfer belt. Particularly, the primary transfer roller tends to slip even more when foreign particles enter the part of the bearing that supports the shaft of the primary transfer roller, and the sliding resistance is increased between the bearing and the shaft of the primary transfer roller.
When the primary transfer roller slips, the back side of the intermediate transfer belt is scraped, as the intermediate transfer belt is made of rubber which is a softer material than metal. As a result, abrasion powder is generated, which is electrically fused with the primary transfer roller. If the primary transfer roller slips on the intermediate transfer belt in a state where the abrasion powder is fused with the primary transfer roller, the abrasion powder forms a film on the primary transfer roller. When such a film is formed, the resistance of the primary transfer roller increases, which makes the primary transfer electric field become insufficient and decrease the transfer efficiency. Accordingly, the image quality becomes degraded over time.
One approach for achieving stable transfer efficiency from the start and over time is to increase the primary transfer bias along with the passage of time. However, this requires complex control methods, which may increase the cost of the apparatus. Another approach is to attempt to scrape off the abrasion powder which has electrically fused with the primary transfer roller by having a cleaning blade abut against the primary transfer roller. However, the abrasion powder which is electrically fused with the primary transfer roller firmly adheres to the metal roller, and thus cannot be effectively removed with a cleaning blade.